Can we stop bowel cancer before it starts?

Despite all the recent hype aspirin is still no ‘magic bullet’ for preventing bowel cancer. For Bowel Cancer Awareness Month we are asking why this might be. We also look at simple ways you could reduce your risk of getting bowel cancer.

Just how preventable is bowel cancer?

Bowel cancer is the world’s third most common cancer and it's also one of the most preventable. Research shows that over half of all bowel cancer cases in the UK could be prevented each year, and the vast majority of avoidable cases are linked to lifestyle.

But what about drugs? Chemoprevention research focuses on finding and developing drugs to prevent cancer before it starts. And when you're talking about chemoprevention of bowel cancer, most people think of aspirin.

Does aspirin reduce the risk of bowel cancer?

Over a hundred years old and with many supposed benefits, aspirin is often touted by the media as the world’s foremost super drug. Increasing evidence does actually seem to suggest that regular low doses of aspirin taken over a length of time might reduce some people’s risk of developing bowel cancer.

Data collected from a very large and long-running study in the US published in JAMAOncologyjust last month backs this up. Their results suggest that regular aspirin could decrease bowel cancer risk by as much as 19 per cent which is encouraging news but these findings come with a few caveats too. For example, like many chemoprevention studies, this study was observational, not a randomised controlled trial (see why this is a problem below). NHS choices gives a brilliantly thorough critique of the data in this paper here.

Even if aspirin does reduce the risk of bowel cancer, the safety of taking it over such a long time is still very much in question. Aspirin use is linked to some serious side-effects including ulcers and stomach bleeding, worsening of asthma and even strokes. Which is why the 'benefit-risk balance' for aspirin still tips the wrong way for most people.

Authorities in the US last year released draft guidelines cautiously recommending low-dose aspirin for certain very specific patient groups at risk of both cardiovascular disease and bowel cancer. However in the UK, NICE has so far refrained from issuing direct guidance about the issue. And right now no medical organisation recommends taking low dose aspirin purely to reduce the risk of bowel cancer.

But this aspirin debate has been around for years, so why do we still not have a definitive answer?

So what is the answer?

As always, it’s never that simple. With chemoprevention, you’re talking about finding a drug that relatively healthy people can take over extended periods of time. So the drug must be not only effective, but also very safe.

This means the benefit-to-risk ratio has to be highly skewed towards the benefit, with less of the risk. Side-effects must be minimal, preferably non-existent. Nobody wants to take something that makes them feel ill to prevent a disease that might not happen anyway. This is the inevitable conundrum of designing chemopreventative drugs. Virtually all drugs, including aspirin, have unwanted effects.

One way to improve the chances of chemoprevention research success is to focus only on patients who have increased risk of developing the disease, perhaps because of a genetic trait which would help shift the benefit-risk ratio. Put simply, people are more likely tolerate side-effects to prevent a near-guaranteed cancer.

This is the strategy Worldwide Cancer Research funded scientist Dr Louis Vermeulen and his team in the Netherlands are following, with encouraging results. Their work, recently published in Nature Communications, points to a potentially new drug target for preventing bowel cancer in patients with a high risk of developing it.

Properly testing any potential new chemoprevention drug can also become enormously expensive very quickly. Studies have to test whether cancer never develops in people taking the drug. That calls for some very long studies, at very large expense. Which might not prove anything at the end anyway. It’s a real gamble. Add to this the necessity that any finished drug must be very cheap if it is to be taken every day, and you start to see the challenges of chemoprevention research.

This is why many chemoprevention studies, like the one published in JAMAOncology, are observational. Researchers follow people over an extended period of time, and try to make connections between their behaviour and their health. And these studies can tell us a lot. But they don’t prove cause and effect, and they don’t necessarily pick-up all side-effects. What's really needed are more well designed randomised controlled trials. For aspirin, these are on the way.

How can I reduce my risk of bowel cancer right now?

Unfortunately not all bowel cancer is preventable. But while we all wait for that magic pill to come along, one of the best ways to potentially reduce your risk is to modify the usual culprits. That means more exercise, avoiding smoking, reducing alcohol, and a better diet. Think high fibre, fruit and vegetables, and maybe not so much processed meat.

Other dietary factors such as calcium and vitamin D have also been linked to reduced bowel cancer risk in some patients. However the jury is still out on this. It’s the usual story- some studies seem to show a positive association in some people, while others show no link at all. What’s for sure is that more research is needed. Ongoing research initiatives like the European Prospective Investigation into Cancer and Nutrition (EPIC) study are helping to tease out the answers.

Bowel cancer screening won’t stop you getting bowel cancer but it will likely pick it up earlier. This makes it easier to treat and can reduce your risk of dying from the disease. In the UK regular bowel cancer screening is offered to everyone between the ages of 60 to 74 (from age 50 in Scotland).

So should I take aspirin to reduce my risk of bowel cancer?

Evidence suggests low-dose aspirin might help prevent bowel cancer in some carefully controlled contexts, but it's certainly not for everyone. The risk of long-term side-effects make it unlikely that we'll all be encouraged to take aspirin purely to reduce the risk of bowel cancer. If you want to know if aspirin might help you then talk to your doctor. Your best bet is still to stay healthy, and get regularly screened as you get older.

Further information:

You can read more discussion about the use of aspirin to prevent any cancer here.